Various environmental laws regulate the discharge or disposal of waste materials into natural streams, municipal sewers, land surfaces, or underground reservoirs since these materials usually contain hazardous or undesirable concentrations of contaminants. Generally, these environmental regulations prohibit the concentration levels of the contaminants (pollutants) from exceeding specific limits which have been determined to ensure a relatively safe environment. The disposal of materials with contaminant levels that exceed these limits is a major problem of growing complexity particularly in view of ever increasing amounts of such materials for disposal and in view of stricter state and federal environmental regulations.
There are many industrial and commercial sites in need of decontamination either because of ongoing operations that generate hazardous waste materials that must be disposed of or because of accumulated waste materials that have not been processed to reduce the contaminants to meet the proscribed limits. The clean-up or remediation of contaminated industrial sites in particular has heretofore presented problems primarily because of the expense and time involved and the tremendous energy requirements that accompany the methods that have been available.
The conventional methods for the cleanup of contaminated sites include: (1) dewatering of sludges, treating the liquid phase, and land disposing the solid phase (filter cake); (2) vaulting in place using slurry walls and stabilization agents; (3) incineration; and (4) thermal desorption. Methods (1) and (2) can be eliminated if the material is covered by EPA Land Disposal Restrictions (LDR) which require that these LDR waste materials meet the applicable treatment standard before subsequent placement in or on the land. If LDR applies, incineration is usually the selected method of disposal. However, incineration is very unpopular with the public often creating additional environmental concerns. For example, incineration in the presence of water and oxygen may result in the formation of dioxins/furans which are known to be extremely toxic and in all likelihood are more hazardous than the initial contaminant that was incinerated. As a result, incineration permits for off-site processing are often either unavailable or very difficult to obtain. Moreover, even when such permits are obtained, on-site incineration may add up to 50% to 300% of additional expenses to the cleanup costs, due to large volumes of combustion gases that will require air handling and treatment equipment such as water quenchers, dust collectors, and water scrubbers. Moreover, the wastes (e.g.,ash) from the incineration process may require disposal in a secured land fill. Since incineration generally results in little volume reduction, disposal costs for the waste ash can be significant. Off-site incineration can add additional costs associated with packaging and transporting the waste materials to the incineration site. Off-site incineration can add up to 30% to 50% of additional costs to the on-site incineration costs. Additionally, off-site incineration can add months to the completion of the cleanup due to low incineration capacity nationwide. Both on-site and off-site incineration can result in residual wastes that are covered by RCRA, resulting in additional encapsulation costs for disposal of the residual wastes generated by the incineration.
The provision of a method for cleaning up sludges or contaminated soils or other contaminated solid waste materials that does not involve on-site or off-site incineration is an increasingly critical need in the art. The problem is particularly acute with industrial sites that contain lagoons, i.e., sludge holding ponds because of the high water content of the sludges, typically requiring dewatering steps before incineration.
One method for decontamination of contaminated sites is thermal desorption, a physical separation process employed for the removal of organics from many types of solid materials such as sludges, filter cakes, and soil materials which is typically carried out using a direct fired rotary drum followed by a baghouse, thermal oxidizer (afterburner or incinerator for gases), water quench to cool the gases, packed scrubber, and stack for emission of gases. Additionally, various systems have been proposed that use direct or indirect heat exchange to achieve bed temperatures of 200.degree. to 1000.degree. F. with oil or combustion gas as the transfer medium. Residence times typically depend on contaminant vapor pressures, sludge conditions, operable variables, and applicable remediation criteria. Residual treatment requirements typically include those for oversized reject, offgases and water. Particulate control systems are also required. Offgas treatment typically includes both thermal oxidation (at temperatures above 1,600.degree. F.) to burn the organics and scrubbing units to remove chemicals such as HCl. Such off-gas treatment is generally significant in terms of cost and meeting regulatory requirements. Although thermal desorption is a proven method for the removal of organics, regulatory agencies often require significant treatability testing and other measures (e.g., test burns) to ensure that there are no emissions problems and that the organics are not transformed into more toxic products as a result of the thermal desorption or offgas treatment processes.
Novel and improved methods utilizing thermal desorption treatment methods are described and claimed in copending U.S. patent application Ser. No. 055,428 entitled METHOD FOR TREATMENT OF CONTAMINATED MATERIALS WITH SUPERHEATED STEAM THERMAL DESORPTION AND RECYCLE, and U.S. patent application Ser. No. 055,433 entitled METHOD AND APPARATUS FOR SOIL REMEDIATION WITH SUPERHEATED STEAM THERMAL DESORPTION AND RECYCLE, both filed concurrently and commonly assigned herewith, the disclosures of which are incorporated by reference.
In U.S. Pat. No. 4,581,144, the subject matter of which is also incorporated herein by reference , there is disclosed a method for treating sludges that result from the chemical treatment of spent sulfuric acid with lime, and in particular a method for solubilizing the solids and reducing the color and total organic content of the treated material to a predetermined level. Such method is highly effective for the stated purpose but still involves a certain amount of incineration. The present invention takes advantage of an embodiment of the patented process in that impounded sludges are subjected to the steps of fluidization, dilution, acidification, clarification, carbon adsorption, and reacidification as in said patented method. However, it has now been found that the steps of blending a portion of recycled bottoms with spent carbon and incineration, previously thought to be essential, may be eliminated. As a result, a vastly improved method may be provided.
Prior to the present improved method, and those of said copending applications, no known method has been available for the cleanup of contaminated solid materials, without incineration, that is portable and compact, multi-stage, optionally continuous, at least as cost effective as the nonincineration conventional methods mentioned above, and significantly more cost effective than methods which involve incineration.